The trouble is, the International War Crimes Tribunal have said they can only try crimes committed IN war itself; they have no jurisdiction on how a decision/decisions to go to war are made.
He may get impeached by parliament under an old law (last used 1806, I think) and would face 'trial by parliament' where a simple majority would convict him. However, the punishment is not likely to imprisonment - I'm sure they'll look after their own, as it were, even though he's not the most popular person in Westminster.....